1. Field of the Invention
This invention relates to the art of corrosion prevention and more particularly, to corrosion preventive resin compositions which are useful, for example, in corrosion prevention of external surfaces of reinforced concrete structures or buildings in seaside regions. The resin compositions have good adhesion, hardenability on wet surfaces, water vapor barrier properties, and flexibility.
2. Description of the Prior Art
In recent years, there have been gradually revealed damages of reinforced concrete structures or buildings (hereinafter referred to simply as RC structures or buildings) in seaside regions due to the injury from salt. Such damages include cracks, blisters, flakings, exposure of reinforcing rods, and the like. Thus, the need of taking a measure to meet the situation has now become pressing. For instance, in order to maintain and use constructed bridges over a long term without any repair works, bridges in seaside regions where environmental conditions are bad, have been constructed of cement concrete (PC structures), called "maintenance-free" concrete, since the first half of 1960. Although the PC bridges are designed to have a lifetime of 50 years, their damages have already become considerable.
The considerable injury of the PC bridges from salt has been seen in the districts from Hokkaido to the side of the Sea of Japan and of the whole islands of Okinawa in the regions of from the coastal splash zone to 200-300 m from the zone.
The reason why the RC structures are injured from salt in the coastal regions is considered as follows. At the initial stage of the hydration action in concrete, excess water is present in concrete and forms capillary interstices, from which it is released. On the contrary, after passage of several tens of hours, moisture in the air is absorbed from the capillary interstices to continue the hydration action.
Calcium hydroxide, which is a hydrate product of concrete, and free lime in cement are, of course, soluble in sea water and have a tendency of being soluble in rainwater. Accordingly, these materials are dissolved out through the capillary interstices, causing the neutralization phenomenon of concrete to occur and also the airtightness to lower.
In the splash zone where salt injury takes place, concrete structures are repeatedly dried and wetted on the surfaces thereof, so that excess water in the concrete is removed by evaporation, and water containing salt is absorbed in the concrete, thereby permitting the salt to be accumulated. This cycle is repeated with the result that an oxide film on the surface of steel rods, which are strongly alkaline (pH of about 12.5), is attacked and corrosion begins.
The corrosion reaction may depend on the degree of oxygen feed, the concentration of salt, the temperature, and the humidity. The main route of salt incorporation into concrete may be divided into two groups including one group in which salt is incorporated from concrete materials, and the other group in which after hardening, salt is incorporated into concrete from outside such as by the spray of seawater, sea breeze and the like. The first group is the case where sea sand is used. The content of salt is prescribed as particularly indicated in explanations of Road and Bridge Regulations, i.e. with reinforced concrete, the content of chlorides, calculated as NaCl, should be 0.1% or below of the weight of cement. The salt damage, which we treat here, is the latter case where salt penetrates into concrete such as by sea spray, sea breeze and the like.
When reinforcing rods have once been corroded, the volume is caused to expand to an extent of 20 times larger than an initial volume. The pressure of the expansion is as high as about 300 kg/cm.sup.2, and causes the concrete to be cracked from inside, resulting in breakage of the concrete (tensile strength 50 kg/cm.sup.2).
In order to prevent the damages due to the salt injury, there was proposed a method as described, for example in Japanese Laid-open Patent Application No. 57-201444. In the method, coating materials comprising synthetic resins which have a glass transition point not lower than 0.degree. C., are applied onto the surfaces of RC structures to form a coating film on the structure surfaces. The coating film is designed to have at least certain levels of water vapor and air proofness. However, the coating material has low adherence to concrete and thus, the film tends to come off by the action of waves, flying stones. In addition, the coating material is in the form of an emulsion and does not show any curability or hardenability on wet surfaces. This is not advantageous in practical applications. Although epoxy resin coating materials have good bonding force or adherence and hardenability on wet surfaces, they are not flexible with an attendant problem that such material cannot follow cracks as will be produced in concrete by temperature change, earthquake and mechanical vibrations.